# Evaluation of Alabama phosphorus index using edge‐of‐field monitoring data

## TL;DR

This study evaluates and improves the Alabama phosphorus index to better predict phosphorus loss from agricultural fields.

## Contribution

The first evaluation and refinement of the Alabama P-index using field data, leading to a modified version with improved accuracy.

## Key findings

- The original Alabama P-index showed weak correlations and directional inaccuracies with measured phosphorus loads.
- A modified P-index with updated parameters demonstrated significant correlations and accurate risk alignment.
- Component-based P-indices require further adaptation for Alabama's agricultural systems.

## Abstract

Phosphorus index (P‐index) was developed to assess field vulnerability to phosphorus (P) loss and guide P management decisions. The original structure of the P‐index was additive, and with continued refinement, multiplicative and component‐based indices were developed. Alabama adopted the additive version in early 2000; however, the tool was never tested for its performance. The objectives of this study were to (i) evaluate the Alabama P‐index using edge‐of‐field P loss data, (ii) test if multiplicative (Tennessee) and component‐based (Georgia) P‐indices perform better, and (iii) improve and test the performance of a modified Alabama P‐index. We evaluated the performance by examining the strength and directional relationship between P‐index scores and annual P loads. The Alabama P‐index showed weak correlations (r < 0.50) between risk scores and measured dissolved reactive phosphorus (DRP), total particulate phosphorus (TPP), and total phosphorus (TP) loads. Additionally, directional inaccuracies were observed, indicating that the index misclassified the relative risk of P loss. Further, we evaluated multiplicative and component‐based indices but found similar discrepancies between predicted risk scores and actual P loading. Subsequently, we modified the Alabama P‐index by replacing soil test P with the phosphorus saturation ratio and substituting the underground outlet system factor with the timing of P application. Minor adjustments to weighting factors were made. The modified P‐index demonstrated statistically significant correlations (r > 0.51) and directional alignment with DRP, TPP, and TP loads, suggesting it can serve as a reliable interim tool for assessing P losses. Future research should focus on restructuring and validating a component‐based P‐index tailored to Alabama's agricultural systems.

The Alabama phosphorus index (P‐index) risk category showed poor alignment with measured P loads, indicating directional inaccuracies.The modified Alabama P‐index showed directional accuracies and can be used as an interim tool for P loss risk assessment.Component P‐index shows promise and warrants further restructuring to suit Alabama's specific conditions.

The Alabama phosphorus index (P‐index) risk category showed poor alignment with measured P loads, indicating directional inaccuracies.

The modified Alabama P‐index showed directional accuracies and can be used as an interim tool for P loss risk assessment.

Component P‐index shows promise and warrants further restructuring to suit Alabama's specific conditions.

Phosphorus loss from agricultural fields contributes to water quality deterioration of freshwater bodies. To address this issue and assess the risk of phosphorus runoff, the phosphorus index was developed as a management tool. In Alabama, the phosphorus index was implemented in the early 2000s; however, it was never evaluated whether the tool accurately captures phosphorus loss risk. The objective of this study was to assess how well the risk categories defined by the Alabama phosphorus index align with actual phosphorus losses observed from agricultural fields. We identified that the Alabama phosphorus index risk rating was poorly aligned with actual edge‐of‐field phosphorus loss, which indicated the need for its refinement. We made the first attempt to modify the parameters in the current phosphorus index and compared it with edge‐of‐field phosphorus losses. The modified version is better aligned with phosphorus loss risk and can be an interim tool for P loss risk assessment in Alabama farms.